Tetrabromobisphenol-A is 4,4'-isopropylidenebis(2,6-dibromophenol). It is a widely used commercial fire retardant. There have been numerous publications on how it can be made. Hennis, U.S. Pat. No. 3,234,289, describes a process in which bisphenol-A (i.e. 4,4'-isopropylidenebisphenol) is placed in a water-alcohol mixture and liquid bromine is added at 22.degree.-28.degree. C. followed by reflux. Majewski et al., U.S. Pat. No. 3,363,007, discloses a process for brominating bisphenol-A in a mixture of water and an alkyl ether of a lower glycol.
Asadorian et al., U.S. Pat. No. 3,546,302, discloses a bromination process conducted in a two-phase solvent having an aqueous phase and an organic phase.
Montanari et al., U.S. Pat. No. 3,868,423, discloses the bromination of isopropylidenebisphenol with liquid bromine and gaseous chlorine in a methanol solvent. Janzon et al., U.S. Pat. No. 3,929,907, discloses the bromination of bisphenols in the presence of aqueous hydrogen peroxide.
Brackenridge, U.S. Pat. No. 4,013,728, teaches a process for brominating bisphenol-A in aqueous acetic acid followed by a heating step. Jenkner, U.S. Pat. No. 4,036,894, discloses bromination of bisphenol-A in acetic acid with recycle of the mother liquor and addition of alkaline or alkaline earth metal acetate.
Production of tetrabromobisphenol-A by dissolving bisphenol-A in methanol and adding liquid bromine is an effective way to make tetrabromobisphenol-A but the product contains various impurities which detract from its commercial value. These impurities include bromophenol, dibromophenol, tribromophenol and hydrolyzable impurities. Thus, a need exists for a process that would lower the amount of these impurities. In pending U.S. application Ser. No. 778,710, filed Sept. 23, 1985, it is reported that the amount of impurities in tetrabromobisphenol-A can be sharply decreased from about 4 weight percent down to about 0.2 weight percent by adding a solution of bromine in methanol to a solution of bisphenol-A dissolved in methanol. This represents a significant advancement in the art. However, there are certain disadvantages inherent in the practice of this process. For example, the bromine-methanol solution cannot be pre-mixed very far in advance of when it is to be reacted with the bisphenol-A since shortly after addition of the liquid bromine to the alcohol, the temperature of the solution rises rapidly to a point which exceeds the boiling point of bromine. At this point, the bromine vaporizes and loss of bromine reactant results. This situation can be ameloriated to a certain degree by cooling the liquid bromine-methanol solution (e.g. under 10.degree. C.) prior to its use or by blending the methanol and bromine in-line just prior to its introduction into the methanol-bisphenol-A solution. However, the former procedure adds an additional step to the process which adds to the cost of producing the tetrabromobisphenol-A product since it requires the use of additional equipment and the latter procedure is not always convenient or available. Accordingly, a welcome contribution to the art would be the provision of a process for making tetrabromobisphenol-A in the same or higher yields and purity as obtained by the aforedescribed liquid bromine in methanol process without the disadvantages of having to use a solution of liquid bromine in methanol as part of the process.